When do you think it's important to establish a web presence?Yesterday.
(1) If you want to, yes. Keep it professional so that when editors/publishers google you, it will make you look good. Professional does not mean it has to be business-like. A well-written slice of life blog is sufficient. This personalizes you to readers. Regular blog visitors are likely to buy your book when it comes out.
Before you ever have hope of being published(1), after acceptance of your manuscript(2), or when the book comes out(3)?
(2) Yes. For sure, get started on something now. Your publisher may have tips or suggestions for you.
(3) This is too late. You want it up and going no later than the day your book hits the shelves.
Do you think a blog is sufficient for a web presence?
Before you're published, yes. After you're published, you need a professional website: www.yourname.com. Doesn't have to be fancy but it must be attractive.
See also info on blogs, and here.